Slugs are Snails Too
Posted on Dec. 21, 2023 / Invasive Species, Nursery, Plant Pests / Subscribe 0
By: Darcy Yogi
Most people don’t think much of snails and slugs, other than “gross.” Many species we see around our communities today are invasive pests and major vectors of the Rat Lungworm parasite (Angiostrongylus cantonensis). However, a morning spent with Dr. Norine Yeung, Bishop Museum Malacology Curator, quickly dissolved this frame of thinking. Dr. Yeung emphasizes that slugs are shell-less snails, therefore anything we deploy for pest slug control will directly affect native snails too. So, it’s critical to be aware of what snails are around you before you control pests in your nurseries and backyards.
Figures 1-3. These images show how snails can have shells of many shapes and sizes to no visible shells at all. However, it’s important to know that regardless of shell, snails and slugs have the same body parts. Illustration source: Sami Chang.
We can’t conserve what we don’t know
Dr. Yeung has spent her career passionately working to conserve Hawaiʻi’s native land snails, which along with other mollusks have the highest recorded extinction rate of any major animal taxonomic group in the world. Hawaiʻi is no stranger to this kind of loss being a biodiversity hotspot, which means high extinction rates and high species diversity. From 21 colonization events, Hawaiian land snails went through a spectacular radiation over 500 million years and exploded to over 750 known species.
Unfortunately, Dr. Yeung admitted that our snails are going extinct faster than we can study them with about 500 species already listed as threatened and endangered. Regardless of that fact, she has been a part of the Hawaiʻi land snail survey for over 10 years and their findings amazed everyone. Before 2012, it was falsely assumed that 90% of all native land snails were extinct but after surveying that number was brought down to 65%. By 2023, their team detected over 350 known and some new native species many of which remain highly threatened.
Figure 4. Dr. Norine Yeung presented at her snail workshop on the importance of our native land snails or kāhuli. She emphasized how we cannot begin to talk about invasive snails without understanding our native snail biodiversity first. The photo shows her highlighting native snails as a food source for the invasive and failed bio-control, rosy wolf snail (Euglandina rosea). Source: Darcy Yogi, BIISC.
Losing ecological and cultural connections
Our land snails live in the forests, in the canopy, and under the leaves. Unless we know where to look, then we may never see them. This almost “invisible” loss of some of our tiniest forest creatures makes it harder for people to grasp the enormity of the impact. Our native snails are just as beautiful as they are functional because they perform vital nutrient cycling for our low-nutrient tropical forests. Native snails are essential cleaners of the forest, scraping off fungus, breaking down organic waste, and excreting “fertilizer packets.” Through those processes, snails have helped to maintain the health of the forests and down to the reefs.
Figure 5. Our native land snails live in the highest canopies of our native forests, down to sub-canopy ferns and shrubs, and even within the herbaceous and forest floor layers. This has implications for invasive snail control because our native snails also travel across the forest floor, which puts them at risk of any pesticide or snail bait that is deployed. Illustration source: Sami Chang
When we think about the impact and loss of snails, the ecological losses rival the cultural losses as many people today are not even aware we still have kāhuli or native snails left. This is heartbreaking as snails were once the voice of the forest. Many old stories and songs talk about when snails were plentiful enough to sing in the canopy while the winds sifted through the trees. Their colorful shells were highly prized jewels of the forest as they glistened in the sunlight and their image was often associated with romance. The Hawaiian name kāhuli means to turn or change, which could refer to how it moves, its swirling shell growth, and its ability to change forms. This significance of land snails in Hawaiian culture spotlights the compounding loss of the intimate relationship society once had with our snails.
The native land snails left today face many threats like land use change, pollution, species overexploitation, climate change, invasive species, and disease. However, Dr. Yeung and her team are working hard to conserve these fierce survivors by conducting snail surveys, describing newly discovered species, sharing engaging outreach, and advocating for more conservation funding. Without this kind of work, it is predicted that our native land snails would lose 11 genera and about 100 species within the next 5-10 years.
Figure 6. If you’re interested in how you can do more for our native snails after reading this article, please volunteer with your local conservation, forestry, or museum agency. To learn more, visit the Bishop Museum’s Land Snail Conservation Program. Illustration source: Sami Chang.
The rise of invasive snails
Unlike native snails, their non-native counterparts have seen dramatic increases in the number of species and individuals due to the globalization of trade via ships and planes. Most of these new species are unintentionally introduced through agricultural and horticultural products like Christmas trees, ornamental plants, and flower bouquets. Dr. Yeung noted specifically that the worst snail invaders come from temperate areas like Europe and the Pacific Northwest because of the large elevational range these species can inhabit.
Dr. Yeung and colleagues conducted nursery surveys across the State and recorded over 40 non-naive snail species across 62 facilities. They found a maximum of 17 species at one facility, recorded 8 new species to the State, and 27 new island records. Results from these surveys are foundational to understanding a snail pest’s biology, behavior, habitat range, and its impacts on nurseries, native forests, and community health. However, Dr. Yeung notes it has been about a while since the last time these surveys could be funded. So, researchers are having to rely more on community reporting to understand what snail pests are present and where and what their impacts are.
I found a weird snail, now what?
Slugs and snails are common nursery and garden pests with voracious appetites. Unlike our native snails which feed on fungus and detritus, our pest species will munch down on plants leaving behind a gooey slime trail and damaged leaves. Some can carry a higher parasite load than others (like semi-slugs), but you should treat all slugs and snails with caution. We recommend always handling with gloves or tools, never with your bare hands.
If you need help identifying a snail (which includes slugs!), please first check out the Big Island Invasive Species Committee (BIISC) Slugs and Snails Pest page (biisc.org/pest/slugs-and-snails) to view a visual guide of common snail pests. To learn more about priority snails, slugs, diseases, and other pests, you can explore the Plant Pono Pest Grid. Within each pest info page, you can dive into the details of each pest’s impacts, identifying characteristics, distribution, and best management practices.
Like with all integrated pest management, the first step is identifying what pest you’re dealing with. If you are on Hawaiʻi Island, you can submit a clear picture of the specimen at multiple angles with something in the photo for scale like a coin or pencil. The photo can then be submitted to our slug reporting app here. BIISC will work to identify the slugs and get back to you. On other islands, take the animal to your Hawaiʻi Dept. Of Agriculture or Invasive Species Committee. Once you can confirm the snail or slug you’re dealing with, then extension and invasive species staff can help you determine appropriate control methods.
Darcy Yogi lives in Hilo, HI, and works as the Invasive Plant Prevention Technician for the Big Island Invasive Species Committee. She helps to support the Plant Pono program, which focuses on preventing the spread of invasive plants and insect pests within the nursery trade on Hawaiʻi Island.
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